Recently, some combustion engines have been controlled to idle at a reduced rotational speed for the purpose of emission control or reduction of fuel consumption. With the combustion engine operating at such an idle rotation set at such a reduced value, the rotational speed will decrease to cause an improper operating condition when the engine is subjected to a small amount of load caused for example by an electric load of a headlight, actuation of an air compressor for an air conditioning unit, or circulation of cooling water to cool a heat exchanger of a heater. To avoid this, some combustion engines have an idle-up mechanism for opening a throttle valve of a carburetor to increase an idle speed. (See Japanese Patent Laid-Open Publication 55-5437).
There is known another combustion engine which is provided with a fuel-cut mechanism for cutting a slow speed fuel flow for the carburetor to prevent an over-fuel condition of the air-fuel ratio during a reduced speed operation. (See Japanese Utility Model Publication 58-42590).
It has been a customary practice that such idle-up and fuel-cut mechanisms are operated independently. The fuel-cut mechanism operates when a throttle-opening detector switch detects that the carburetor throttle valve is opened within a predetermined degree of throttle opening during the reduced speed operation of the engine.
However, the throttle-opening detector switch is kept detecting that the throttle valve is opened beyond the predetermined degree of throttle opening if the degree of throttle opening for idle-up is wrongly set to a value greater than the predetermined degree monitored by the detector switch. In this case, the fuel-cut mechanism fails to cut the slow speed fuel flow, thus causing an improper operation thereof, with the result that an air-fuel mixture of an objectionable over-fuel ratio is supplied to the engine and produces incomplete combustion gases, which in turn will mingle with the emission control catalyzer to cause an objectionable burning of the latter.